Transposase activity was thought to be extinct in humans because DNA movement can be deleterious in higher organisms, resulting in genomic instability and perhaps malignancy. We isolated a human transposase protein termed Metnase that had preferential endonuclease activity for supercoiled DNA. We therefore explored its role in decatenating tangled DNA. DNA replication results in intertwined sister chromatids that must be untangled, or decatenated, before chromatid separation at anaphase. When absent, catastrophic chromatid breakage can occur, and the multiple breaks can result in inter- chromosomal end joining and subsequent translocation. Therefore, cells actively monitor the decatenation of intertwined chromatids. The presence of catenated DNA usually induces a cell cycle arrest, termed the decatenation check point, before mitosis. It has been previously shown that bladder and lung cancers do not properly arrest at the decatenation cell cycle checkpoints. We also found that acute leukemias do not arrest at the decatenation checkpoints. The decatenation checkpoints are activated when ATR senses catenated chromosomes and then signals for the cell to arrest before mitosis. Topo II1 is the essential decatenating enzyme for physically untangling chromatids. Beyond that, little is known about the molecular mechanism of this recently described phenomenon. We found that Metnase, rather than mediating the mobility of DNA segments, increased chromosome stability by markedly enhancing the rate of DNA decatenation by Topo II1. Increasing Metnase levels increased resistance to the Topo II1 inhibitors ICRF-193 and VP-16. Based on these fundamental findings, we hypothesize that Metnase plays a role in hematopoietic decatenation, and assists in progression through the decatenation cell cycle checkpoint. This application will characterize the mechanism by which this takes place by addressing three questions: 1) What is the mechanism by which Metnase enhances Topo II1 decatenation activity? 2) Is the decatenation activity of Metnase activated by phosphorylation signals?,3) Does Metnase mediate decatenation in hematopoietic malignancies such as leukemia? Deciphering the mechanism by which Metnase functions in decatenation could lend insight into the mechanism of leukemogenic translocations, why leukemia fails to appropriately arrest at the decatenation check points, and resistance of some leukemias to Topo II1 inhibitors. PUBLIC HEALTH RELEVANCE: We have isolated a novel protein termed Metnase that helps chromosomes untangle, and thereby prevents their breakage during cell division. Preventing such chromosome breakage can reduce the mutations that cause many forms of cancer. The cancer cell, however, can subvert Metnase, and use it to resist the actions of DNA damaging chemotherapy.